limbs) developed as enable the organism to place itself in harmony with a complex environment, but to further increase this harmony the structures themselves are rendered highly variable; whereby extreme development of this or that structure or organ occurs in stress of need, but useless developments which would only encumber the organism do not occur. For this reason the muscles in a clerk's arm do not develop to the same extent as the muscles in a blacksmith's arm.
This evolution of corporal variability corresponds, as will be seen later, to an equal or greater evolution of mental variability, so that physically as well as mentally the action of Natural Selection has been to produce great individual powers of varying; but the variations produced under this power are not transmitted, only the power to vary and to acquire under appropriate stimulation similar variations is transmitted; thus, to take an example, no infant is born of fully developed parents whose limbs or mental powers can attain a similar full development without stimulation from the environment.
As regards physical variability, with which alone we are at present concerned, in general each individual of a species in his "normal" development nearly attains its limits (e.g. beyond the normal development of muscle, but a relatively small further development is possible); the limit being fixed by the struggle between Natural Selection and Atavism, the one operating to increase variability, the other to diminish it, to bring it to a condition of ancestral non-variability.
But all the structures do not "normally" approach the limit to an equal degree, horns, hair (in animals in which hair is merely ornamental, and in those living in an equable climate, when variability under stimulation in this respect is not essential to survival), teeth, nails, &c., if we except the effects of disease, which cannot
